Force, Motion, and Newton's Laws
Grade 7 · Science · Study Guide
Understand how forces cause objects to move, stop, and accelerate through Newton's three fundamental laws.
Motion is everywhere — but what actually causes it? Isaac Newton figured out the rules in 1687, and they still hold up today in everything from car crashes to rocket launches. His three laws of motion describe the relationship between forces and the movement of objects.
Newton's First Law of Motion says that an object at rest stays at rest, and an object in motion stays in motion at the same speed and direction, unless acted on by an unbalanced (net) force. This is called inertia — the tendency of objects to resist changes in their state of motion. A soccer ball sitting still won't move until you kick it. A hockey puck sliding on frictionless ice would keep going forever. In real life, friction acts as the unbalanced force that slows things down.
Newton's Second Law of Motion says that force equals mass times acceleration (F = ma). The more force you apply to an object, the more it accelerates. But the heavier (more massive) the object, the harder it is to accelerate. This is why it takes more force to push a full shopping cart than an empty one. Force is measured in Newtons (N).
Newton's Third Law of Motion states that for every action there is an equal and opposite reaction. When a rocket engine pushes gas downward (action), the gas pushes the rocket upward (reaction). When you jump off a boat, you push the boat backward. These action-reaction forces always act on different objects and are always equal in size but opposite in direction. Understanding these three laws lets engineers design cars, spacecraft, sports equipment, and buildings that work safely and efficiently.
| Law | Statement | Real-World Example |
|---|---|---|
| 1st Law (Inertia) | Objects resist changes in motion | Wearing a seatbelt when the car stops suddenly |
| 2nd Law (F=ma) | More force = more acceleration; more mass = less acceleration | Pushing a heavy cart requires more force than a light one |
| 3rd Law (Action-Reaction) | Every action has an equal and opposite reaction | Rocket propulsion; jumping pushes the Earth (very slightly) |
Key terms
- Force
- A push or pull that can cause an object to change its motion.
- Inertia
- The tendency of an object to resist any change in its state of motion.
- Newton's First Law
- An object stays at rest or in motion unless acted on by a net (unbalanced) force.
- Newton's Second Law
- F = ma; force equals mass times acceleration.
- Newton's Third Law
- For every action force, there is an equal and opposite reaction force.
- Net force
- The overall combined force acting on an object after all forces are added together.
Flashcards
- Newton's First Law
- Objects at rest stay at rest; objects in motion stay in motion — unless a net force acts on them. (Law of Inertia)
- Newton's Second Law
- F = ma. The net force on an object equals its mass times its acceleration.
- Newton's Third Law
- For every action force, there is an equal and opposite reaction force acting on a different object.
- Inertia
- Resistance to changes in motion; larger mass = greater inertia.
- Net force
- The vector sum of all forces acting on an object.
- Acceleration
- A change in velocity (speed or direction) over time; measured in m/s².
Practice questions
Which of Newton's laws states that F = ma?
- First Law
- Second Law
- Third Law
- Law of Gravity
Show answer
Answer: Second Law
Newton's Second Law (F = ma) mathematically relates force, mass, and acceleration. It tells you that a larger force produces greater acceleration, and a larger mass requires more force to accelerate.
An object at rest will stay at rest unless an unbalanced force acts on it.
Show answer
Answer: True
This is Newton's First Law — also called the Law of Inertia. Objects do not change their motion on their own; they need an unbalanced force to start, stop, or change direction.
The tendency of an object to resist changes in its state of motion is called ___.
Show answer
Answer: inertia
Inertia is the property of matter that keeps objects doing what they're already doing. Heavier objects have more inertia and are harder to get moving (or to stop).
A rocket moves forward because gases shoot out the back. This is an example of which law?
- First Law
- Second Law
- Third Law
- Law of Conservation
Show answer
Answer: Third Law
Rocket propulsion is Newton's Third Law in action. The rocket engine pushes hot gases backward (action); the gases push the rocket forward (reaction) with equal force.
Doubling the mass of an object (with the same force applied) will double its acceleration.
Show answer
Answer: False
From F = ma, if mass doubles and force stays the same, acceleration is HALVED (a = F/m). More mass means less acceleration for the same applied force.
Fun facts
- Newton reportedly developed his ideas about gravity after thinking about why an apple fell straight down from a tree — though the 'apple hitting his head' story is likely a myth.
- An astronaut in space still has mass (and inertia) — they just don't feel weight because there is no surface pushing up on them.
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